The invention relates in general to manufacturing methods and in particular to manufacturing methods for enhancing residual stresses. The following non-patent literature is referred to in the specification and is expressly incorporated by reference herein:    [1] Bauschinger, J., 1881, “Ueber die Veranderung der Elasticitatagrenze and dea Elasticitatamoduls verschiadener Metalle”, Zivilingenieur, Vol 27, columns 289-348.    [2] Parker, A. P., Troiano, E., Underwood, J. H. and Mossey, C., 2003, “Characterization of Steels Using a Revised Kinematic Hardening Model Incorporating Bauschinger Effect”, ASME Journal of Pressure Vessel Technology, Vol 125, pp. 277-281.    [3] Lemaitre, J. and Chaboche, J.-L., 1990, Mechanics of Solid Materials, Cambridge University Press.    [4] Parker, A. P., 2001, “Autofrettage of Open-End Tubes—Pressures, Stresses, Strains and Code Comparisons”, ASME J Pressure Vessel Technology, Vol 123, pp. 271-281.    [5] O'Hara, G. P., 1992, “Analysis of the Swage Autofrettage Process”, US Army ARDEC Technical Report ARCCB-TR-92016, Benét Laboratories, Watervliet Arsenal, N.Y. 12189, USA.    [6] Parker, A. P., O'Hara, G. P. and Underwood, J. H., 2003, “Hydraulic versus Swage Autofrettage and Implications of the Bauschinger Effect”, ASME Journal of Pressure Vessel Technology, Vol 125, pp. 309-314.    [7] Underwood, J. H., deSwardt, R. R, Venter, A. M., Troiano, E., Parker, A. P., “Hill Stress Calculations for Autofrettaged Tubes Compared With Neutron Diffraction Residual Stresses and Measured Yield Pressure and Fatigue Life”, Paper PVP2007-26617, Proceedings of PVP2007, 2007 ASME Pressure Vessels and Piping Division Conference, Jul. 22-26, 2007, San Antonio, Tex.    [8] Iremonger, M. J. and Kalsi, S. K., “A Numerical Study of Swage Autofrettage”, 2003, ASME Journal of Pressure Vessel Technology, Vol 125, pp. 347-351.    [9] Parker, A. P., 2004, “A Re-Autofrettage Procedure for Mitigation of Bauschinger Effect in Thick Cylinders”, ASME Journal of Pressure Vessel Technology, 126, pp. 451-454.    [10] Parker, A. P. and Kendall, D. P., 2003, “Residual Stresses and Lifetimes of Tubes Subjected to Shrink Fit Prior to Autofrettage”, ASME Journal of Pressure Vessel Technology, Vol 125, pp. 282-286.    [11] Paris, P. C. and Erdogan, F., 1963, “A Critical Analysis of Crack Propagation Laws”, Journal of Basic Engineering, Trans ASME, Vol 85, pp. 528-534.    [12] Underwood, J. H., Moak, D. B., Audino, M. A. and Parker, A. P., 2003, “Yield Pressure Measurements and Analysis for Autofrettaged Cannons,” Journal of Pressure Vessel Technology, 125, pp. 7-10.    [13] Troiano, E., Underwood, J. H., deSwardt, R. R., Venter, A., Parker, A. P. and Mossey, C, 2007, “3D Finite Element Modeling Of the Swage Autofrettage Process Including the Bauschinger Effect”, ASME PVP2007-ICPVT12 Conference, Paper PVP2007-ICPVT12-26743, July 22-26, San Antonio, Tex., USA.
Prior to normal use, many engineering components and structures are subjected to overloads in excess of their design operating level. Examples of such overloads are “shakedown” of a bridge structure; hydraulic or swage autofrettage of a pressure vessel (including gun barrels); and mandrel enlargement of rivet holes (including aircraft structures).
In general, the purpose of such overloads is to cause the stresses within the material(s) to behave in an inelastic fashion at design-critical locations and thereby, on removal of initial overload, to induce advantageous residual stresses at or near the critical locations. These residual stresses subsequently serve to mitigate the stresses due to normal operation and thereby improve fatigue lifetime and/or improve fracture resistance and/or inhibit re-yielding. For example, in the case of a typical pressure vessel, the use of autofrettage can increase the fatigue lifetime of a tube with pre-existing crack-like defects by approximately one order of magnitude.
Many materials exhibit the Bauschinger effect [1], [2]. The Bauschinger effect serves to reduce the yield strength in compression as a result of prior tensile plastic overload (or vice-versa, when compression precedes tension). It is often assumed that the Bauschinger effect is associated with the pile-up of microscopic dislocations at grain boundaries and the associated creation of microscopic zones of residual stress [3]. The reduction of yield strength after load reversal is of importance because, on removal of the overload, critical regions experience high values of reversed stress. This may approach the magnitude of the yield strength if the unloading is totally elastic. If, because of the Bauschinger effect, the combination of stresses exceeds some yield criterion, the component or structure will re-yield, thus losing much of the potential benefit of overloading.
The loss of residual compressive hoop stress has been quantified for the case of hydraulically autofrettaged open-end tubes [4]. Such tubes are overloaded during an autofrettage process involving extremely high bore pressures applied to the length of the tube; this is usually termed “hydraulic autofrettage”. The ratio of the wall thickness which behaves plastically during autofrettage to the total wall thickness is termed the “overstrain”. As a rule of thumb, for typical diameter ratios and overstrain levels, “ideal” residual compressive hoop stress at the bore is reduced by 30% by Bauschinger effect and associated effects. As a result, the fatigue lifetime of a typical tubular steel pressure vessel subjected to 80% overstrain which does not exhibit Bauschinger effect may be more than one order of magnitude greater than the same pressure vessel which does exhibit Bauschinger effect. Hence, if it were possible to eliminate the deleterious impact of the Bauschinger effect, the lifetime of the component would be very significantly increased.